Turalyon will be Alliance, but this is one of those wars where the Horde and Alliance will join up against a greater foe - like in Warcraft 3. Not doing so would be irresponsible and retarded.
He will lead the military side of the 'Army of Light', with a crew of seasoned veterans that have fought the Legion with him on his journeys. These include humans, high elves, dwarves, brown orcs, and draenei mostly. Both Horde and Alliance will be able to work with him against the Legion, but afterwards he will report back to the King of Stormwind (if he survives).
Alleria is on the same track, but putting them both together would be a waste of resources. She will have her own faction of High Elves, attempting to convert the Blood Elves and Silvermoon back to their original allegiances. Some will join her, others wont. Voila, High Elves to the Alliance, with their main base in Northeron. She'll also have an interesting story going on with Sylvanas and Vereesa.
As a bonus, Illidan will return. Him and the demon hunter faction will represent a more sinister and rebellious side of Azeroth against the legion. You can fight with him or Turalyon, but you share the same enemy.